GTOFBOD

Hey guys, I'm having a problem diagnosing a misfire code in a 2006 lq4 express van I'm using for a swap. Ok so the problem started out after I bought the van and hooked my code reader to it (out of curiosity), only code that came up was p0300 old immature, I cleared the code then rechecked a day later and it was back. It has never set the engine light on constantly, but did start to blink once when free revving the engine. I then decided to scan with hptuners, everything looks normal, maf, map, o2s, etc. except my fuel trims are way positive, like 15 to 20 range, and it does show some random misfires on both banks on the misfire counter. Now the van runs great, feels strong, no funny noises, etc. So far I have cleaned the maf and replaced air filter with aftermarket in stock box. Fuel pressure hangs out in the 60-62 range, but does bleed down a few pounds maybe after ten minutes. Also checked for vacuum leaks, didn't find any but will check again. Also, I did unplug the maf, went for a drive, and the trims came back inline, but it ran like a total dog, it was smooth and no misfires, but lacked power. I'm kinda stumped, and I really don't want to throw parts at it. Anyone have any ideas?

mrvedit

Fuel trims of 15-20% indicate something is seriously out of wack.
Correct me if I am wrong, but when you disconnect the MAF, you are forcing open loop operation, the fuel trims are ignored (reset?) and fueling is running just off the VE table.
Since there is no 15% adjustment, I'm not surprised it lacked power.

Assuming the engine is stock, I would start by checking the fuel pressure which is supposed to be 58psi. I think that low pressure would give you those symptoms.

Note: I am unsure about my advice because I am unsure that disconnecting the MAF forces open-loop operation. Mostly I'm just trying to get the conversation going and hopefully an expert can confirm or correct me.

GTOFBOD

Thanks, no prob. I always thought when you unplugged the maf, the pcm would reference the ve table, which is making me think maf, I would think even if referencing the ve table the trims would still be way postive if there was a vacuum leak. Fuel pressure is 62 at idle and increases slightly under load. I also pulled the number 7 and 8 plugs. Both were bone dry with and had redish color on the insulator.